For five months, the taskforce worked on unravelling the encrypted email addresses and Tor software which Falder used to share sick images with other paedophiles and to blackmail victims into performing perverted tasks.

A still taken from surveillance footage of Matthew Falder on a train.

Uncovering Falder's true identity was no easy task.

He used around 70 online identities to contact around 200 people from Cornwall to northeast Scotland, Wales to Northern Ireland, as well as in the United States and Canada.

There was no money trail to follow. Falder's currency was kudos in his perverted community rather than cash.

He did not conform to the stereotype of a computer geek living alone and friendless in a grotty dump.

Falder was from Knutsford, Cheshire, taught geophysics at Birmingham University, had a PhD from Cambridge and was sociable and well-liked.

He was said to be the life and soul of the party and his unsuspecting girlfriend doted on him.

Falder had an affluent upbringing and a top class education.

His Twitter feed revealed his love of dinosaurs, baking, table tennis and 3D-printing.

But on the dark web he was a VIP member of child abuse forums and he used popular and legal sites such as Gumtree to con young people into sending him nude photos of themselves - before the blackmail began.

Mr Long, who led the NCA investigation, said: "What was a surprise to us was that here was a very respectable man of good standing, with no previous convictions, working in a very reputable institution.

"But we also knew we were dealing with someone who was intelligent, committed and very sadistic. "The level of dedication to remaining anonymous was significant. He was almost living a double life."

For three months NCA detectives tailed him, even filming him using his laptop on a train journey. Footage of Falder's arrest in June 2017 shows him asking: "What is it I am supposed to have done?"

When his crimes were read out he said it sounded "like the rap sheet from hell".

Police seized his computers and a USB stick but its information had been double-encrypted to hide its vile contents.

Computer experts working on both sides of the Atlantic cracked them and discovered a file marked "BM". Mr Long said: "We presumed that was his blackmail folder.

"In there were hundreds of images of his victims which were absolutely horrific.

"Over the last four years we had been tracing these victims all over the world, interviewing them on video, suddenly the jigsaw pieces came together.

"Once we had arrested him and got that information from his devices we could conclusively say that Dr Matthew Falder was Evilmind."

Evilmind had first come to the attention of the police in August 2013, when the FBI closed down "the worst website in the world", Hurt 2 The Core.

One of its vilest contributors was a man calling himself, among 70 other aliases, "Inthegarden" - who posted blackmail images of a teenage girl.

Mr Long explained: "We believed he was in Britain but we could not be sure."

For the next two years investigators tracked Inthegarden's web trail and discovered the same person was posing as a female artist on legitimate sites such as Gumtree.

They also ascertained that 666devil, Evilmind and Inthegarden were the same person.

Falder's conviction was hailed as a "watershed moment" this week by NCA staff, but The Sun can reveal that already other forums on the dark web are filling with users trying to work outhow to beat the cops.

Jamie Bartlett, best-selling author of The Dark Net, said: "They will be discussing what mistake Falder might have made to get caught and be working out how not to repeat it.

"It is getting easier to access child porn, but harder for the authorities to trace the offenders.

"The dark net has become more user-friendly. And you don't need to be a tech nerd like Falder any more to use it.

"It takes GCHQ, dozens and dozens of police officers, working with several other police forces around the world, several years to catch just one person.

"I am afraid there are many people like him out there who are not getting caught. There aren't those kind of resources for all of them.

"I am sure GCHQ would not have got involved in this had it not needed the kind of data analysis and number-crunching that they are capable of. They use all different techniques to de-anonymise people."

But what this case shows is that the authorities will keep going until they get you.

"It is great that the police got Falder because it sends out a signal that they will come after you," Bartlett said.

Mr Long said last night: "There is nowhere a child abuser can hide that we will not follow.

"Everything we have learned from the investigation and conviction of Matthew Falder will be redeployed in earnest against any offender hidden within the dark web.

"I'm not going to say the dark web is cracked, it isn't. But with a significant effort the taskforce will continue to focus on dangerous offenders and bring them to justice."